1. Field of the Invention
The present invention relates to an oxygen sensor, particularly an oxygen sensor for measuring contents of oxygen contained in gases, such as exhaust gases exited mainly from internal combustion engines, boilers, or the like, and a method of producing the same.
2. Related Art Statement
Heretofore, concentrations of oxygen in exhaust gases exited from internal combustion engines of automobiles, boilers or the like are detected based on the principle of an oxygen concentration cell using oxygen ion conductive solid electrolytes such as zirconia ceramics for controlling an air-fuel ratio or combustion state of such internal combustion engines.
Oxygen sensors of this type of oxygen concentration detector adopt a structure of providing desired electrodes as sensing elements on the inner and outer surfaces of a bottom cylinder or plate shaped solid electrolyte body, exposing the inner electrode to the atmosphere to use it as a reference electrode exposed to a reference gas of a reference oxygen concentration, and exposing the outer electrode to an exhaust gas to be measured to use it as a measuring electrode. By detecting an electromotive force generated by a difference of oxygen concentrations of the reference electrode and the measuring electrode, an oxygen concentration in the exhaust gas is measured.
In conventional oxygen sensors of this type, oxygen sensor elements comprise, as major components, a desired shape of an element body made of a desired oxygen ion conductive solid electrolyte, and a plurality of electrodes arranged on the surface of the element body. Such sensor elements have drawbacks in that the measuring electrode which is the outer electrode is worn and damaged by the action of the exhaust gas of a high temperature to deteriorate its sensing function and cause other problems. Therefore, in order to protect such measuring electrode a porous protective coating layer made of spinel or the like is formed on the measuring electrode to a desired thickness by a plasma thermal spray method or a print sintering method, etc.
However, problems arise even in this sensor element having the protective coating layer. For example, if it is mounted as an oxygen sensor on an exhaust tube of an automobile, and an inferior fuel containing an organic silicon, etc., is used inadvertently or an engine packing or the like containing a silicon compound is used and combusted by the engine, fine particulate shaped or gaseous organic and inorganic silicon compounds are formed and passed through the pores of the protective coating layer to adhere on the surface of the ceramics, or adsorbed on the surface of the measuring electrode or the interface between the measuring electrode and the ceramics to cause deterioration of the sensor output or deterioration of the response, which problems are called silicon poisoning of oxygen sensors.
Meanwhile, prevention of the measuring electrode from reaching the silicon compounds by means of thickening the protective coating or making the pores small is also conceivable. This method prevents fine particles of the silicon compounds from reaching to the measuring electrode. However, the method is not always a satisfactory method since the gaseous silicon compounds can not completely be prevented from reaching the measuring electrode, the initial response is deteriorated due to the thick protective layer and the fine pores, and the fine pores may be become clogged such that the response is deteriorated gradually during the use of the oxygen sensor.